1. Field of the Invention
The present invention relates to a spark-ignition cylinder injection engine which directly injects fuel into a cylinder using a high-pressure injector.
2. Description of the Related Art
In general, a cylinder injection engine is capable of accurately controlling parameters such as a fuel injection timing and an amount of fuel injection. Therefore, selective switching between homogeneous combustion (premixed homogeneous combustion) and stratified charge combustion can be easily performed in accordance with an operating state of the engine. The stratified charge combustion may be performed, for example, in a fast idle period immediately after the start of the engine. Accordingly, ignition timing can be retarded without reducing ignition quality and a large amount of thermal energy generated by the combustion can be supplied to an exhaust side. As a result, catalyst can be quickly activated.
Japanese Unexamined Patent Application Publication No. 2002-155748 (hereinafter referred to as Patent Document 1) discloses a cylinder injection engine of this type in which a cavity for guiding fuel injected from an injector toward a spark plug is provided at the top of a piston and a mask wall that prevents a part of an intake port from being opened by an intake valve is provided on the intake port. An amount of lift of the intake valve is variably controlled to selectively switch between the homogeneous combustion and the stratified charge combustion. According to the technique described in Patent Document 1, in an operating range in which an engine rotational speed and an engine load are both relatively low, a maximum lift of the intake valve is set to a high lift so that an injection flow rate is reduced. Accordingly, the cavity at the top of the piston effectively functions, and the stratified charge combustion can be achieved. In addition, in an operating range in which the engine rotational speed and the engine load are both relatively high, the maximum lift of the intake valve is set to a predetermined amount of lift at which the intake valve is not moved beyond the mask wall so that the injection flow rate is increased. Accordingly, a strong tumble flow is generated in the cylinder, and the homogeneous combustion (homogeneous combustion) can be achieved. In an operating range in which the engine rotational speed and the engine load are both further higher, the maximum lift of the intake valve is set to a high lift (or an intermediate lift at which the intake valve is moved slightly beyond the mask wall), so that a large amount of intake air is sucked into the cylinder. Accordingly, uniformly mixed fuel-air mixture can be provided.
However, when the cavity for stratifying the fuel-air mixture is formed at the top of the piston as in Patent Document 1, the cavity serves to reduce the persistence of the tumble flow. Therefore, as a result, there is a risk that the fuel consumption efficiency during the homogeneous combustion will be degraded.
The reduction in the persistence of the tumble flow can be compensated for by using devices such as a high-tumble port or a tumble-generating valve (TGV). However, these devices may serve as a flow-passage resistance, which makes it difficult to increase the maximum engine output.